Nanotechnology can be defined as materials or devices engineered at the molecular level. Within this category are polymer nanocomposites, which are a class of materials that use molecular sized particles for reinforcing the polymer matrix, e.g., the reinforcing filler possesses one or more dimensions on a sub-micrometer scale. These materials blend a nanofiller with a polymer to produce a composite with equal or better physical and mechanical properties than their conventionally filled counterparts but at lower filler loadings.
Due to the surface area available with nanofillers, polymer nanocomposites offer the potential for enhanced mechanical properties, barrier properties, thermal properties, and flame retardant properties when compared to conventionally filled materials.
One class of polymer nanocomposites uses a nanofiller material that is based on the smectite class of aluminum silicate clays, a common representative of which is montmorillonite. Although naturally occurring and synthetic variations of this basic mineral structure can be used to make nanocomposites, the structure should allow the exchange of interlayer inorganic cations, such as Na+ or Ca2+, with organic cations, such as alkylammonium cations, if property enhancements are to be achieved. The silicate platelets consist of a central octahedral aluminate structure surrounded on either side with a tetrahedral silicate structure. Iron or magnesium occasionally replaces an aluminum atom, rendering an overall negative charge. This charge is counterbalanced by the inorganic cations which reside between the sheets, holding them loosely together. The exchange of interlayer inorganic cations with organic cations increases the spacing between the silicate sheets, as well as improves the compatibility of the filler and the resin system, thereby facilitating exfoliation.
When exfoliated properly, these layered silicates have size dimensions approximately 1 nm thick by about 50 to 2000 nm long. This leads to aspect ratios on the order of about 50 to 2000. This value is quite high compared to the aspect ratio of conventional fillers such as talc (aspect ratio ˜1) and glass fibers (aspect ratio ˜20). Due, at least in part, to this high aspect ratio, there is the potential to obtain properties equal to or greater than conventionally filled materials but at much lower filler loadings, for example, between about 2% and about 5%. Conventionally filled materials require about 20% to 30% loadings to achieve equivalent property enhancement.
For optimum reinforcement properties, not only is good exfoliation desirable, but also good distribution of the silicate layers throughout the resin, and good compatibility between the polymer resin and the filler.
Exfoliation of the nanofiller reinforces the resins which results in enhanced physical and mechanical properties. Traditional processes use external compatibilizers to make the nanofiller less polar and increase its miscibility with non-polar olefinic resins. However, one of the drawbacks of current nanocomposite materials is a potential lack of development of a high degree of exfoliation (dispersion) of the nanofiller material.